A conventional flat speaker includes a transparent vibratory sound panel mounted to the window frame of the casing of an acoustic device having a display assembly and adapted to protect a display screen such as a liquid crystal display, a suspension by which the sound panel is supported on the window frame, and an exciter secured to the inner surface of the sound panel at a position separately from the display screen and adapted to cause the vibratory sound panel to vibrate in response to an electric signal (see, for example, Japanese patent disclosure No. 2002-533957). Reference will first be made to FIGS. 14 to 16 wherein one example of such a conventional flat speaker is shown. FIG. 14 is a plan view, partly broken away, of a known cell phone. FIG. 15 is a partial sectional view taken on the line XV-XV in FIG. 14. FIG. 16 is a wave graph showing the sound characteristic of the flat speaker shown in FIG. 14.
In FIGS. 14 and 15, the reference numeral 20 denotes a cell phone with a display assembly. The cell phone 20 has a casing 21, a printed circuit board 22 and a display screen 23, such as a liquid crystal display, secured to the printed circuit board 22. Although not shown, the display screen 23 includes a holder for securing a back light, the liquid crystal display and other elements to the printed circuit board 22. A vibratory sound panel 2 is mounted on a window frame 21a of the casing 21 and protects the surface of the display screen 23. The vibratory sound panel 2 includes a transparent display region 2a and an exciter mount region 2b. A printed layer 2c is attached to the outer periphery of the display region 2a and the exciter mount region 2b. A space 8 is defined between the display region 2a and the display screen 23. An exciter 3 is mounted to the inner surface of the exciter mount region 2b and adapted to cause the vibratory sound panel 2 to vibrate in response to an electric or acoustic signal. A suspension 4 is in the form of a frame and supports the vibratory sound panel 2 on the upper surface of the window frame 21a. The suspension 4 is made of a cushioning material, such as sponge, with an adhesive applied to its opposite sides (for example, “Poron L32” sold by Nitto Kagaku Kogyo K.K.). The suspension may alternatively be in the form of a double-sided adhesive or a similar adhesive. The suspension 4 has a function to flexibly hold the vibratory sound panel 2 and dampen vibrations of the vibratory sound panel.
Referring to FIG. 15, a flat speaker 1 is composed of the vibratory sound panel 2, the exciter 3 and the suspension 4. A seal 6 is in the form of a frame and is made of a cushioning material such as sponge. The seal 6 is sandwiched between the inner surface of the window frame 21a and the display screen 23 and extends around the top edge of the display screen 23. The flat speaker 1 is supported on the casing 21 solely by the suspension which extends around the peripheral edge of the sound panel 2. This arrangement enables the vibratory sound panel 2 to be readily vibrated by the exciter 3. The vibratory sound panel 2 is thus capable of producing sound with a wide range of frequencies and a high level of sound pressure. In FIG. 16, the sound characteristic denoted by an indication “speaker” is determined by first inputting a large input signal to the exciter and then, measuring the sound pressure and the frequency of sound at a point separated from the flat speaker by 10 cm. The sound characteristic denoted by an indication “receiver” is determined by first inputting a small input signal to the exciter and then, measuring the sound pressure and the frequency of sound as if the ear of a user is placed in close contact with the vibratory sound panel.
The known flat speaker 1 has a space 8 above the display screen 23 and a space 9 adjacent the exciter 3. The space 8 is communicated with the space 9. If dust is attached to the exciter 3 and its surrounding parts, such dust may possibly enter the space 8. As the display surface of the display screen 23 is visible through the display window of the casing 21 and the space 8, dust, if introduced into the space 8, deteriorates the esthetic appearance of the display screen. To this end, attempts have been made to isolate the space 8 from the space 9, as shown in FIG. 17. Specifically, the display screen 23 is located inside the window frame 21a. The seal 6 is sandwiched between the display screen 23 and the sound panel 2. The seal 6 serves as a dust seal to isolate the space 8 from the space 9. The seal 6 is substantially identical in thickness to the suspension. However, such an arrangement prevents free vibration of the vibratory sound panel 2 and also, deteriorates the sound characteristic of the flat speaker due to a lack of sound pressure, as shown in FIG. 18.
It is, therefore, an object of the present invention to provide an acoustic device and a flat speaker which can improve the esthetic appearance of a display screen without losing its sound characteristic.